(a). Field of the Invention
The present invention relates to a metal-semiconductor field effect transistor (MESFET, referred to as simply "FET" hereinafter) and, more particularly, to an improvement of an intermodulation distortion characteristic of a FET.
(b). Description of the Related Art
A high speed FET implemented by a compound semiconductor such as GaAs has been used in a power amplifier and an oscillator for a microwave range. The high speed FET is requested to have a reduced intermodulation distortion characteristic due to the recent, remarkable increase in number of transmission channels or amount of data. The term "intermodulation distortion" means an undesirable signal component, in an output of the FET, having a frequency of mf.sub.1 .+-.nf.sub.2 when a pair of signals having frequencies of f.sub.1 and f.sub.2 are supplied to the inputs of the FET, wherein m and n each represents an integer. A tertiary or third-order component of the intermodulation distortion is generally represented by "IM3", which includes 2f.sub.1 -f.sub.2 and f.sub.2 -f.sub.1 frequency components and are dominant distortion in the output signal. The intermodulation distortion is known to originate from an odd-order non-linearity of the FET, as described by J. A. Higgins and R. L. Kuvas, in "Analysis and Improvement of Intermodulation Distortion in GaAs Power FETs", IEEE Trans., Microwave Tech. vol. MTT-28, pp. 9-17, January 1980.
In order to reduce the intermodulation distortion in an output of a FET, reduction of non-linearity in the transfer characteristic of the FET is attempted. Examples of such attempts include a pulse doping technique for a carrier node profile of an active layer of the FET to maintain the transconductance (gm) of the FET as a constant value, as proposed by R. A. Pucel, in "Profile Designs for Distortion Reduction in Microwave Field-Effect Transistors", Electron, Lett, 1978, 14, pp. 205-206. In those attempts, a class A operation is generally assumed in order to obtain a most improved operation in terms of intermodulation distortion.
However, a class AB or F operation should be considered for a highly efficient operation of the FET, wherein the drain current on the operational point is set at a tenth of the maximum drain current of the FET. Accordingly, if the drain current amplitude is relatively small or at an output level 10 dB lower than the maximum output level, the drain current is subjected to clipping by a pinch-off. In this case, even if the transfer characteristic is determined such that the transconductance is maintained as a constant value, the intermodulation distortion characteristic is not improved due to the clipping.
FIG. 1 shows a calculated intermodulation distortion characteristic wherein ordinate and abscissa represent IM3 and output level (P.sub.out), with the transconductance being maintained at an ideally constant level during a class AB operation. The output resistance, maximum drain current and bias current are 0.5 ohm, 10A and 1A, respectively. As shown in the figure, IM3 rises as high as -22 dBc when the output level P.sub.out is around 30 dBm.
In short, the conventional FET has a drawback in that IM3 is high due to the clipping by the pinch-off in a class AB or F operational mode of the FET.